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doi:10.2204/iodp.proc.348.102.2015 LithologyAt Site C0002 (Holes C0002N and C0002P), cuttings data, core data between 2163 and 2217.5 mbsf in Hole C0002P, and LWD data (including NGR, resistivity imaging, ultraseismic caliper, and sonic data) were used to identify lithologic boundaries and units. Methods applied to core description during Expedition 348 draw upon the protocols of IODP Expeditions 315 (Expedition 315 Scientists, 2009a) and 338 (Strasser et al., 2014a), whereas methods applied to description of cuttings rely upon procedures established during Expedition 319 (Expedition 319 Scientists, 2010b), in particular the Cuttings Cookbook (Center for Deep Earth Exploration [CDEX], 2012). Cuttings samples from Holes C0002N and C0002P were described based on the examination of a 70 cm3 aliquot of bulk cuttings. Descriptions included
Depths reported for cuttings are on the MSF depth scale (Table T1). Core samples were described based on
Depths reported for cores and samples are on the CSF depth scale (Table T1). Figures F4 and F5 show the graphic patterns for the lithologies encountered in core and cuttings during Expedition 348. Macroscopic observations of cuttingsCuttings typically occur as small fragments of rock, generally 0.25–8 mm in size of various lithologies, produced during drilling. Cuttings were taken for the first time in IODP operations during Expedition 319 (Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010). Sampling and analysis of cuttings follow the Cuttings Cookbook (CDEX, 2012) developed during Expedition 319, with some additions and modifications. Cuttings were separated by sieving by laboratory technicians into rock-chip fractions of different sizes (0.25–1, 1–4, and >4 mm). However, at shallow depths solid fragments from the formation are sometimes suspended in drilling mud and mixed with trace amounts of clay-bearing drilling additives (e.g., bentonite). Rigorous separation of drilling-related mud from formation cuttings is not always possible, especially in the case of very soft cuttings. This hampers quantification of the true clay content. The procedure for separating cuttings from drilling mud and the division into different sizes is explained in the Cuttings Cookbook (CDEX, 2012). Cuttings were collected at 5 m intervals at 875.5–2330 and 1941.5–3058.5 mbsf in Holes C0002N and C0002P, respectively, with samples analyzed and described every 10 m. Based on general visual observations of the bulk cleaned cuttings material, we estimated the relative amount of silty claystone and sandstone; the consolidation state; the shape; and the occurrence of wood, fossils, and artificial contamination (Fig. F5). All macroscopic observations were recorded on visual cuttings description forms and summarized in VCDSCAN in “Supplementary material.” Macroscopic observations of coreWe followed conventional Ocean Drilling Program (ODP) and IODP procedures for recording sedimentologic information on VCD forms on a section-by-section basis (Mazzullo and Graham, 1988). VCDs were transferred to section-scale templates using J-CORES software and then converted to core-scale depictions using Strater (Golden Software). Texture, which is defined by the relative proportions of sand, silt, and clay, follows the classification of Shepard (1954). The classification scheme for siliciclastic lithologies follows Mazzullo et al. (1988). Where applicable in core, bioturbation intensity in deposits was estimated using the semiquantitative ichnofabric index as described by Droser and Bottjer (1986, 1991). The index refers to the degree of biogenic disruption of primary fabric such as lamination and ranges from 1 for nonbioturbated sediment to 6 for total homogenization:
The ichnofabric index in cores was identified with the help of visual comparative charts (Heard and Pickering, 2008). Distinct burrows that could be identified as particular ichnotaxa were also recorded. The Graphic lithology column on each VCD plots all beds that are ≥2 cm thick to scale. Interlayers <2 cm thick are identified as laminae in the Sedimentary structures column. It is difficult to discriminate between the dominant lithologies of silty claystone and clayey siltstone without quantitative grain-size analysis; therefore, we grouped this entire range of textures into the category “silty claystone” on all illustrations. A more detailed description of rock texture was attempted on the smear slide description sheets, which are provided (see smear slides in “Core descriptions”). Separate patterns were not used for more heavily indurated examples of the same lithologies (e.g., silty clay versus silty claystone) because the dividing line is arbitrary. Figure F4 shows symbols for sedimentary structures, soft-sediment deformation structures, severity of core disturbance, and features observed in XRCT images in both soft sediment and indurated sedimentary rock. X-ray computed tomographyXRCT imaging provided real-time information for core logging and sampling strategies. We used a methodology similar to that used during Expedition 316 (Expedition 316 scientists, 2009). XRCT scans were used routinely during this expedition on all core samples. XRCT scanning was done immediately after cores were cut into sections so that time-sensitive whole-round samples (e.g., those for interstitial water) could be included in this screening process. The scans were used to provide an assessment of core recovery, determine the appropriateness of whole-round and interstitial water sampling (e.g., to avoid destructive testing on core samples with critical structural features), identify the location of subtle features that warrant detailed study and special handling during visual core description and sampling, and determine the 3-D geometry, crosscutting and other spatial relations, and orientation of primary and secondary features. Microscopic observation of cuttingsMicroscopic investigations of the washed >63 µm sand-sized fraction using a binocular microscope allowed us to distinguish different minerals in the sand-sized fraction of the sediment; their abundance, roundness, and sorting; and the relative abundances of wood/lignite fragments and fossils. The mineralogy in the mudstone could not be determined because of the small grain sizes of the minerals. The data are summarized in “Lithology” and Figure F8 in the “Site C0002” chapter (Tobin et al., 2015) (see also VCDSCAN in “Supplementary material”). Errors can be large, however, especially for fine silt– and silt-sized fractions. Thus, it would be misleading to report values as exact percentages. Instead, the visual estimates are grouped into the following categories:
Smear slidesSmear slides are useful for identifying and reporting basic sediment attributes (texture and composition) in samples of both cuttings and cores, but the results are semiquantitative at best (Marsaglia et al., 2013). We estimated the abundance of biogenic, volcaniclastic, and siliciclastic constituents using a visual comparison chart (Rothwell, 1989). Cuttings pieces were chosen for smear slide production based upon the dominant lithology present in a given interval. If a distinct minor lithology was abundant, an additional smear slide was made for that interval. For cuttings, we estimated the percentage of minerals observed, normalized to 100%. Smear slide images and scanned smear slide forms of cuttings are presented in the SMEARSLD folder in “Supplementary material.” For core, estimates of sand, silt, and clay percentages were entered into the J-CORES samples database along with abundance intervals for the observed grain types, as given above. Additional observations, including visual estimates for normalized percentages of grain size and mineral abundance, were handwritten on the paper smear slide forms, which were scanned and are included in SMEARSLD in “Supplementary material.” The sample location for each smear slide was entered into the J-CORES database with a sample code “SS.” The relative abundance of major components was also validated by XRD (see “X-ray diffraction”), and the absolute weight percent of carbonate was verified by coulometric analysis (see “Geochemistry”). Smear slides were observed in transmitted light using an Axioskop 40A polarizing microscope (Carl Zeiss) equipped with a Nikon DS-Fi1 digital camera. X-ray diffractionThe principal goal of XRD analysis of cuttings and cores was to estimate the relative weight percentages of total clay minerals, quartz, feldspar, and calcite from peak areas. For cuttings, XRD analysis was conducted on a 10 g subsample of the 1–4 mm size fraction every 10 m. These measurements were also made on the >4 mm size fraction, for comparison. For cores, material for XRD was obtained from a 10 cm3 sample that was also used for XRF and carbonate analyses. All samples were vacuum-dried, crushed with a ball mill, and mounted as randomly oriented bulk powders. Routine powder XRD analyses of bulk powders were performed using a PANalytical CubiX PRO (PW3800) diffractometer. XRD instrument settings were as follows:
In order to maintain consistency with previous NanTroSEIZE results, we used the software MacDiff 4.2.5 for data processing (http://www.ccp14.ac.uk/ccp/web-mirrors/krumm/macsoftware/macdiff/MacDiff.html). We adjusted each peak’s upper and lower limits following the guidelines shown in Table T4. Calculations of relative mineral abundance utilized a matrix of normalization factors derived from integrated peak areas and singular value decomposition (SVD). As described by Fisher and Underwood (1995), calibration of SVD factors depends on the analysis of known weight-percent mixtures of mineral standards that are appropriate matches for natural sediment. SVD normalization factors were recalculated during Expeditions 315 and 338 after the diffractometer’s high-voltage power supply and X-ray tube were replaced (Expedition 315 Scientists, 2009a). The mixtures were rerun at the beginning of Expedition 348 (Table T5). Bulk powder mixtures for the Nankai Trough are the same as those reported by Underwood et al. (2003): quartz (Saint Peter sandstone), feldspar (Ca-rich albite), calcite (Cyprus chalk), smectite (Ca-montmorillonite), illite (Clay Mineral Society IMt-2, 2M1 polytype), and chlorite (Clay Mineral Society CCa-2). Examples of diffractograms for standard mixtures are shown in Figure F6. Average errors (SVD-derived estimates versus true weight percent) of the standard mineral mixtures are total clay minerals = 3.3%; quartz = 2.1%; plagioclase = 1.4%, and calcite = 1.9%. Despite its precision with standard mixtures, the SVD method is only semiquantitative, and results for natural specimens should be interpreted with caution. One of the fundamental problems with any bulk powder XRD method is the difference in peak response between poorly crystalline minerals at low diffraction angles (e.g., clay minerals) and highly crystalline minerals at higher diffraction angles (e.g., quartz and plagioclase). Clay mineral content is best characterized by measuring the peak area, whereas peak intensity may more accurately quantify quartz, feldspar, and calcite. Analyzing oriented aggregates enhances basal reflections of the clay minerals, but this is time consuming and requires isolation of the clay-sized fraction to be effective. For clay mineral assemblages in bulk powders, the two options are to individually measure one peak for each mineral and add the estimates together (thereby propagating the error) or to measure a single composite peak at 19.4°–20.4°2θ. Other sources of error are contamination of mineral standards by impurities such as quartz (e.g., the illite standard contains ~20% quartz) and differences in crystallinity between standards and natural clay minerals. For trace quantities of a mineral and peaks with low intensity, use of negative SVD normalization factors may result in negative values of absolute weight percent. In such cases, we inserted the numerical value of 0.1% as a proxy for “trace.” Therefore, calculated mineral abundances should be regarded as relative percentages within a four-component system of clay minerals + quartz + feldspar + calcite. How close those estimates are to their absolute percentages within the total solids depends on the abundance of amorphous solids (e.g., biogenic opal and volcanic glass), as well as the total of all other minerals that occur in minor or trace quantities. For most natural samples, the difference between calculated and absolute abundance percentage is probably between 5% and 10%. To compound the error, the XRD data from cuttings show effects of contamination by drilling fluid. The severity of these artifacts is especially obvious in the calculated values of percent calcite. Figures and tables are available in “Lithology” in the “Site C0002” chapter (Tobin et al, 2015). X-ray fluorescenceAnalyses were obtained in two modes: analysis of whole-rock powders and scanning of the whole-core surface on some selected intervals. Whole-rock quantitative XRF spectrometry analysis was undertaken for major elements on cuttings and cores. For cuttings, XRF analysis was conducted on a 10 g powdered subsample of the 1–4 mm size fraction every 10 m. These measurements were also made on the >4 mm size fraction for comparison. For cores, material for XRF was obtained from a 10 cm3 sample that was also used for XRD and carbonate analyses. For both cuttings and cores, all samples were vacuum-dried and crushed with a ball mill. Major elements were measured using the fused glass bead method and are presented as weight percent oxide proportions (Na2O, MgO, Al2O3, SiO2, P2O5, K2O, CaO, TiO2, MnO, and Fe2O3). An aliquot of 0.9 g of ignited sample powder was fused with 4.5 g of SmeltA12 flux for 7 min at 1150°C to create glass beads. Loss on ignition (LOI) was measured using weight changes on heating at 1000°C for 3 h. Analyses were performed on the wavelength-dispersive XRF spectrometer Supermini (Rigaku) equipped with a 200 W Pd anode X-ray tube at 50 kV and 4 mA. Analytical details and measuring conditions for each component are given in Table T6. Rock standards of the National Institute of Advanced Industrial Science and Technology (Geological Survey of Japan) were used as the reference materials for quantitative analysis. Table T7 lists the results for selected standard samples. A calibration curve was created with matrix corrections provided by the operating software, using the average content of each component. Processed data were uploaded into an Excel spreadsheet and are shown in “Lithology” in the “Site C0002” chapter (Tobin et al., 2015). XRF core scanningXRF core scanning analysis was performed using the JEOL TATSCAN-F2 energy dispersive spectrometry–based core scanner (Sakamoto et al., 2006). The Rh X-ray source was operated at 30 kV accelerating voltage and a current of 0.170 mA. Data are reported as total counts on the peak and also as semiquantitative oxide weight percent. Semiquantitative analysis was performed using a 200 s accumulation. The following oxides were measured: Na2O, MgO, Al2O3, SiO2, P2O5, K2O, CaO, TiO2, MnO, and Fe2O3. This is the same methodology as the one used during Expeditions 316 (Expedition 316 Scientists, 2009) and 338 (Strasser et al., 2014a). The archive half was scanned because this technique is nondestructive to the core material. Sections 348-C0002P-5R-4, 35–91 cm, and 5R-5, 0–59 cm, were scanned at a spatial resolution of 0.5 cm and an analytical spot size of 1 cm × 1 cm. The scanning line was located along the center axis of the core section. Identification of lithologic unitsIn Holes C0002N and C0002P, we used LWD data (see details in “Logging”) in conjunction with analyses of cuttings and core to identify lithologic units and boundaries. We identified compositional and textural attributes of the formation mainly using gamma radiation data, resistivity and sonic logs, and resistivity images along with macroscopic, microscopic, and mineralogical data from cuttings. After evaluating log data quality through the examination of the potential effects of borehole diameter, borehole conditions, and drilling parameters, we defined units using changes in log responses interpreted to reflect differences in rock properties. For this analysis, gamma radiation and resistivity logs were the main input. Integrated interpretation of all the available logs focused on (1) definition and characterization of units and unit boundaries, (2) identification of composition and physical properties within each unit, and (3) interpretation in terms of geological features (unit boundaries, boundaries, transitions, sequences, and lithologic composition). We interpreted lithologic units within the core, as with cuttings, using a broad suite of data including logs, VCDs, smear slides, thin sections, XRD, XRF, carbonate analysis, and XRCT images. |
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